1. Field of the Invention
The present invention relates to a composition or formulation for improving scorch conditions in the preparation of grafted and/or crosslinked polymers, and of filled plastics, where a silicon-containing compound, a free-radical initiator and a free-radical scavenger are present in the composition. The present invention also relates to polymers and to filled plastics which are obtainable using a formulation of this type, and to appropriate processes for preparing said polymers and plastics, and to items based on these polymers or plastics.
2. Discussion of the Background
It is known that in the preparation of moisture-crosslinkable polymers silanes may be grafted onto polymer chains in the presence of a free-radical generator (FRG) and that the crosslinking by moisture is then carried out after shaping.
It is also known that in the preparation of highly filled polymer/filler systems, i.e. filled plastics, silanes are grafted onto polymer chains in the presence of an FRG, thus bringing about or improving the compatibility of the inorganic and organic components.
The term “scorch” covers the processing period, concluding with the onset of the FRG-initiated reaction, of a polymer mixture which is being grafted and/or crosslinked during preparation or processing, and which comprises FRG. A disadvantage here is that, in particular when using types of polymer susceptible to reaction on contact with free radicals, for example polyethylene (PE) with a narrow molar mass distribution and/or with a high molecular weight, polypropylene (PP), ethylene-vinyl acetate copolymer (EVA) with vinyl acetate (VA) content of from 0 to 40% by weight, ethylene-propylene-diene terpolymer (EPDM, “rubber”), or ethylene-propylene elastomer (EPM), some degree of deactivation of the free radicals takes place through undesired side-reactions before the material leaves the processing plants. The results include low reaction conversions, nonuniform distribution of the reacted starting components, low output rates due to undesired viscosity increase of the polymer material to be processed, and also solid particles which have an adverse effect on product quality.
The crosslinking by moisture of polymers using hydrolyzable unsaturated silanes is used worldwide in producing cables, pipes, foams, etc. Processes of this type are known as the Sioplas process (DE 19 63 571 C3, DE 21 51 270 C3, U.S. Pat. No. 3,646,155) and the Monosil process (DE 25 54 525 C3, U.S. Pat. No. 4,117,195). In the Monosil process, the crosslinking catalyst is added before the first step of processing finishes; whereas in the Sioplas process the crosslinking catalyst is not added until the second step.
During the use of highly filled thermoplastic and/or moisture-crosslinkable polymer mixtures, PE and/or EVA and/or EPDM and/or EPM and/or PP are chemically modified using formulations made from unsaturated silane esters in the presence of FRG, and inorganic fillers may be incorporated beforehand, at the same time or subsequently.
The chemical modification of the polymers takes place via linkage (grafting) of the silane esters to the polymer chain via free-radical addition. In the first stage of the process it is desirable to homogenize the starting materials, and degradation of the FRGs in this phase is undesirable. Temperature-controlled processing controls the decomposition reaction. The temperature of the reaction components and, the reaction mixture within the polymer reactor or extruder can generally only be controlled via the barrel temperature or by way of internal parts of the plant. Effects within the homogenizing zone, for example shear forces, give rise to local temperature peaks which exceed the critical processing temperature of the FRG. Another issue is the actual decomposition curve of the peroxides. The course of the reaction should ideally correspond to a step function. This mode of reaction is applicable only for relatively high temperatures for a particular peroxide used. Since the polymer melt cannot instantly be brought to, and held at, the precise reaction temperature needed, but instead traverses a temperature gradient, the reaction conversion of the FRG is delayed, meaning that the course of the reaction generally takes the form of a relatively flat “S-shaped curve”. The consequence of the two factors is that before the homogenization phase has ended free radicals are produced and in turn cause undesired side-reactions, e.g. C—C linkages of polymer chains, and also local peaks of concentration of grafted silane esters, to mention just a few examples.
These disadvantages have hitherto been countered—without any satisfactory success—by using FRGs of varying decomposition temperature as determined by varying process requirements. Although this shifts the “creeping” FRG degradation, i.e. “prescorch”, to higher temperatures, the unfavorable decomposition curve is substantially unaltered. The term prescorch generally means undesired premature crosslinking during the production process of mixing or creeping.
An example of the detailed discussion of this problem in the context of peroxide-crosslinking of elastomers and polyethylenes is that by J. Groepper (“Scorch-verzögernde Peroxide für die Vernetzung von Elastomeren und Polyolefinen” [Scorch-inhibiting peroxides for the crosslinking of elastomers and polyolefins], GAK 2/1994, Volume 47, pages 83 to 88). Here, too, the FRGs to be used for the crosslinking are first incorporated into a polymer matrix, and then the polymer matrix is crosslinked by increasing the temperature, and the use of what are known as SRFRGs (SR=“scorch-retardant”, i.e. crosslinking-suppressive) is described as an approach for solving the “scorch problem”. EP 0 346 863 A1 and JP-A 09-302 043 disclose the use of hydroquinone derivatives as free-radical scavengers. Hydroquinone derivatives generally have the disadvantage of very modest to poor solubility in vinylsilanes, e.g. vinyltrimethoxysilane (VTMO). The combination, as described in JP-A 09-302 043, of high peroxide contents and hydroquinone as SR component can also impair the processability of the extrusion material. For example, no extrusion trials could be carried out using the mixture described in JP-A 09-302 043, since the mixture caused extruder shutdown because the material became non-extrudable.